Oncology is the branch of medicine that deals with tumors, including the study of their development, diagnosis, treatment and prevention. A tumor is an abnormal growth of tissue resulting from the uncontrolled, progressive multiplication of cells, serving no physiological function. A tumor may be malignant (cancerous) or benign. A malignant tumor is one that spreads cancerous cells to other parts of the body (metastasizes) through blood vessels or the lymphatic system. A benign tumor does not metastasize, but can still be life-threatening if it impinges on critical body structures such as nerves, blood vessels and organs.
Radiosurgery and radiotherapy are radiation treatment systems that use external radiation beams to treat tumors and other lesions by delivering a prescribed dose of radiation (e.g., X-rays or gamma rays) to a target area (region of interest, or ROI) while minimizing radiation exposure to the surrounding tissue. The object of both radiosurgery and radiotherapy is the destruction of tumorous tissue while sparing healthy tissue and critical structures. Radiotherapy is characterized by a low radiation dose per treatment and many treatments (e.g., 30 to 45 days of treatment). Radiosurgery is characterized by a relatively high radiation dose to a tumor in one, or at most a few, treatments. In both radiotherapy and radiosurgery, the radiation dose is delivered to the tumor site from multiple angles. As the angle of each radiation beam is different, every beam passes through the tumor site, but passes through a different area of healthy tissue on its way to the tumor. As a result, the cumulative radiation dose at the tumor is high and the average radiation dose to healthy tissue is low.
Conventional radiotherapy and radiosurgery treatment systems use a rigid and invasive stereotactic (3-dimensional reference) frame to immobilize a patient during a diagnostic/treatment planning CAT (computed axial tomography) scan or other 3-D imaging modality (e.g., MRI or PET scan) that images the region of interest, and during subsequent radiation treatments. The rigid frame is attached to bony structures in the patient (e.g., the skull) so that reference marks on the frame (fiducials) have a fixed spatial relationship with the region to be imaged (e.g., the brain). Subsequently, during treatment, the frame provides points of reference for the location of a radiation beam (or beams). In a conventional radiosurgery system, a distributed radiation source (e.g., cobalt 60) is used to produce a number of simultaneous radiation beams through holes in a custom-machined radiation shield. In a conventional radiotherapy system, the radiation source is a single beam device mounted in a gantry structure that rotates around the patient in a fixed plane of rotation. Every beam passes through the center of rotation (the isocenter) and the patient must be properly positioned or repositioned with respect to the isocenter before each radiation beam is applied
Image-guided radiotherapy and radiosurgery systems (together, image-guided radiation treatment, or IGR treatment systems) eliminate the use of invasive frame fixation by correcting for differences in patient position between the treatment planning phase (pre-treatment imaging phase) and the treatment delivery phase (in-treatment phase). This correction is accomplished by acquiring real-time X-ray images during the treatment delivery phase and registering them with reference images, known as digitally reconstructed radiograms (DRRs), rendered from a pre-treatment CAT scan.
FIG. 1 illustrates a schematic representation of a CAT scanner. As shown in FIG. 1, an X-ray source produces a fan beam of X-rays that travels through the patient and impinges on a detector. While the treatment table is stationary, a cross-sectional image of the patient is obtained by rotating the X-ray source and detector around the patient and scanning a transverse slice of the body from different angular positions. After each cross-sectional slice is complete, the table is advanced (perpendicular to the plane of FIG. 1) and the next cross-sectional slice is obtained. A three-dimensional (3-D) image (CT volume) is obtained by integrating the image data from the slices. The CAT scan is used to develop a treatment plan that calculates the angle, duration and intensity of the X-rays beams needed to deliver the prescribed radiation dose.
A DRR is a synthetic X-ray image produced by combining data from the CAT scan slices and computing a two-dimensional (2-D) projection through the slices that approximates the geometry of the real-time imaging system. The registration process between the DRR's and the real-time X-ray images is designed to correct for translational and rotational misalignments between the reference images and the real-time images.
The accuracy of the registration is limited by the accuracy of the DRR's used for the registration process. The accuracy of the DRRs is limited, in turn, by the resolution of the diagnostic CAT scan. As noted above, a DRR is a synthetic X-ray. A DRR is obtained by integrating tracing lines from slice-to-slice through the CT volume. Compared to X-ray images, DRRs are blurred and some image details may be lost. Thus, the registration process compares high-quality real-time X-ray images to low quality DRR images and the overall quality of the registration is limited by the resolution of the DRR.
FIG. 2 illustrates one potential problem associated with the use of DRR's. In FIG. 2, a tumor mass is shown in close proximity to a blood vessel with an irregularity. The CAT scan consists of a series of cross-sectional slices separated by a series of increments, where the increments serve to limit the total X-ray exposure of the patient to safe levels. When a DRR is generated from the CAT scan data, the image is rendered by linear interpolation between the slices, which washes out details in the rendered image. FIG. 3 illustrates such a rendering of the tumor mass and blood vessel from the data obtained from FIG. 2. In FIG. 3, the irregularity in the blood vessel is lost because the increment between the CAT scan slices is greater than the size of the irregularity. The loss of feature data degrades the effectiveness of any similarity measures between the DRR and the real-time X-ray images used to drive the registration process. The DRR rendering process also causes a loss of density variation data in the increments, which may make it difficult or impossible to track the movement of soft tissue structures during treatment.